OUR1/OsbZIP1-OsPIN9 regulatory module controls auxin-dependent crown root formation in rice

Crown root (CR) development is a major determinant of rice root system architecture and influences nutrient uptake, tiller number, and grain yield. Although polar auxin transport mediated by PIN proteins is critical for CR formation, the transcriptional regulation of specific PIN members remains unclear. Here, we demonstrate a novel regulation of <i>OsPIN9</i> mediated by the bZIP transcription factor OUR1/OsbZIP1. Loss of <i>OUR1/OsbZIP1</i> function significantly promotes CR formation under nutrient-sufficient conditions, and this induction requires upregulation of the auxin efflux carrier <i>OsPIN9</i>, which we identify as a direct target of OUR1/OsbZIP1. Disruption of <i>OsPIN9</i> mostly abolished the enhanced CR phenotype of the <i>our1</i> mutant. Spatially adjacent patterns of upregulated <i>OsPIN9</i> and enhanced auxin signaling were accompanied by the induction of the CR initiation regulators <i>CRL1</i>, <i>CRL5</i>, and <i>OsWOX11</i>, defining an OUR1/OsbZIP1-<i>OsPIN9</i> module that integrates auxin transport and signaling to regulate CR development in rice.